Communication cables employing optical fibers are widely used in the telecommunications industry. In particular, multifiber cables are widely used for long distance telephone communications, interexchange telephone applications, and other telephony and data transmission applications. Fiber optic cables are also being incorporated into cable television networks in place of more traditional coaxial cables. Optical fibers may permit long distances between signal repeaters or eliminate the need for such repeaters altogether. In addition, optical fibers offer extremely wide bandwidths and low noise operation.
A fiber optic cable typically includes a core and an outer protective jacket. A plurality of optical fibers are contained within the core. For a typical fiber optic cable, such as used for long distance communications, the fibers are maintained in a loose-buffered relationship within one or more buffer tubes which define the core and thereby isolate the fibers from strain imparted to the cable. A typical loose-buffered cable, such as available from Siecor of Hickory, N.C. under the designation MINIBUNDLE.TM., includes a series of plastic buffer tubes stranded around a central support member in a concentric layer.
Another type of loose-buffered fiber optic cable is offered by AT&T under the designation LIGHTPACK LXE.RTM.. The AT&T cable includes a relatively large single central buffer tube which contains the plurality of optical fibers, as shown in U.S. Pat. No. 4,844,575 to Kinard et al. One or more longitudinal strength members, such as opposed wires, may be positioned within the overall plastic jacket. See also U.S. Pat. No. 5,138,685 Arroyo et al. and U.S. Pat. No. 5,165,003 to Carter which both disclose conventional central buffer tube fiber optic cables.
Similarly, U.S. Pat. No. 5,125,063 to Panuska et al. discloses a lightweight fiber optic cable having a single central buffer tube and a pair of diametrically opposed strength members. Because of the extended strain window of the cable, wire or glass rods may be used as the strength members, and the typical armor layer may be deleted, especially for aerial installations where a lighter weight is desirable. The cable meets the 600 lb. tension limit as standard for typical fiber optic cables. The strength members of a conventional fiber optic cable typically permit about 600 lbs. of pulling tension during cable installation. However, typically once aerially installed, the strength members are no longer needed since the messenger carries the weight of the installed cable.
A conventional fiber optic cable may be installed along its designated route by directly burying the cable in the soil, by placing the cable within a duct line, or by aerially installing the cable along a series of vertical supports. A combination of such installation techniques may also be employed for a given fiber optic communications system. A fiber optic cable is typically aerially installed along a utility wood pole line, for example, by lashing the cable with a helically wound small gauge solid wire to a preinstalled supporting messenger. The messenger is typically a stranded metallic cable secured by respective clamps at each of a series of wood poles or other vertical supports. The cable is pulled into position adjacent the messenger and the lashing wire is helically wrapped around the cable and messenger. Accordingly, the messenger serves to support the weight of the installed cable.
The sag and tension experienced by a conventional aerially installed fiber optic cable and its supporting messenger are most severe during wind and ice loading conditions when a build-up of ice surrounding the messenger and cable presents a large weight increase. Moreover, wind further increases the loading on the cable because the ice also presents a larger surface area. For a typical aerial installation of a fiber optic cable along a utility wood pole line, the cable is typically installed in the so-called "communications space" on the pole which is below and spaced from high voltage power lines for safety reasons. Accordingly, the amount of sag that may be accommodated is limited to also provide adequate clearance in the area underneath the fiber optic cable.
Fiber optic cables of the self-supporting type are also known, which, by definition requires no separate supporting messenger. For example, a figure-8 self-supporting aerial fiber optic cable is known, as disclosed, for example, in U.S. Pat. No. 5,095,176 to Harbrecht et al. The cable includes a first jacket portion in which a support strand is enclosed, and a second portion containing transmission media which may be optical fibers. Similarly, U.S. Pat. No. 4,763,983 to Keith discloses an all-dielectric, figure-8 fiber optic cable having a glass reinforced plastic rod serving as the integral supporting messenger. As typically installed, the integral supporting messenger is uppermost, while the fiber containing portion of the cable is carried underneath the integral messenger.
Self-supporting aerial cables of the figure-8 configuration are also known for electrical power cables as disclosed, for example, in U.S. Pat. No. 960,291 to Egner et al.; U.S. Pat. No. 3,207,836 to Slechta; U.S. Pat. No. 3,267,201 to Pusey et al.; and U.S. Pat. No. 3,297,814 to McClean et al. Similarly, U.S. Pat. No. 4,378,462 to Arnold, Jr. et al. discloses a figure-8 self-supporting copper wire pair telecommunications cable.
Unfortunately, a figure-8 fiber optic cable may be relatively stiff and difficult to handle, especially during installation. Moreover, because the single strength imparting member, that is, the integral messenger is positioned on one side of the fiber containing portion, the overall cable may have a pronounced asymmetrical and preferred bending direction. This makes handling of the cable even more difficult. Such figure-8 cables are typically only suitable for aerial installation. More particularly, the figure-8 cable could not be universally used for direct burial and duct sections of a typical fiber optic cable system.
Another type of aerial self-supporting fiber optic cable includes a layer of aramid yarn surrounding a circular fiber containing core. Unfortunately, the layer of aramid yarn for a self-supporting configuration is relatively expensive. Moreover, because the cable is relatively lightweight and presents a smooth symmetrical surface to wind, such a cable may be subject to wind-induced motion damage. The aramid yarn fiber optic cable is also typically not suitable for direct burial installation or installation within a duct line primarily because of its expense and lack of rodent protection.